So i finally got my Kohler K321 rebuilt properly this time. I have the crank ground to spec at a machine shop, replaced the piston rings and all gaskets. She fires up fine, runs smooth and quiet (new muffler too) but there is some vapour coming out of the crankcase breather after she warms up to operating temp.

I checked the oil and it does smell like raw gas. The oil is also visibly not the same viscosity as when it was put in.

I'm not sure how to troubleshoot this problem. Where do I start? I noted some moisture around the fuel pump body where it is attached to the crankcase.

It sounds like the fuel pump diaphragm has a hole in it & pumping fuel into your crankcase. DO NOT run it until you check this out! If you can take the pump loose from the block and leave fuel lines hooked up, pulling the pump out of the block, you may see right away it is leaking. If not apparent, try pumping the pump lever to see if it shows leakage then. Either a new OE pump, or in my opinion, switching to a low pressure electric fuel pump would be the best solution.

A "Mr Gasket" pump model "S42" is a good choice for such applications.

Yes, I agree with Daniel, cease running it until you get the fuel leak found & fixed. Oh, and don't pull the lunkhead move I did, I found my problem & then started it. ran it for about 10 seconds before my eyes fell on the stuff to drain the oil out...

I'll blame it on ADD, but it's really just stupidity. It survived my idiotic move and is still running the "Roadside Mower" up at my parents. They learned a long time ago not to use the good mower to find roadside litter and rocks. Found a hammer that way...

It sounds like the fuel pump diaphragm has a hole in it & pumping fuel into your crankcase. DO NOT run it until you check this out! If you can take the pump loose from the block and leave fuel lines hooked up, pulling the pump out of the block, you may see right away it is leaking. If not apparent, try pumping the pump lever to see if it shows leakage then. Either a new OE pump, or in my opinion, switching to a low pressure electric fuel pump would be the best solution.

A "Mr Gasket" pump model "S42" is a good choice for such applications.

Daniel, I had already disconnected the outbound fuel line when I pulled the fuel pump off. I did disassemble the pump and there didn't seem to be any holes in the diaphragm, but I'll do it again and post pics so you can see. On a related note, I pulled apart my carb and noticed that the choke plate was bent right where the baffle is over the holes. I tried to remove the plate and both of those brass screws cracked off... now i need to buy a new choke shaft

Man, when it rains it pours!!! Worst part is, I just bought 2 Honda 200S ATCs to putter with. Now my garage is full of fixables... ugh.. lol

I have had luck twice in the past very by carefully drilling out those screws. Go slow and do not let it heat up, at some point, you will see the other end move and then they will screw on out the other side.

Here's the diagphragm pic. No holes, but under it on the lever side, there was a nice puddle of gas/oil mixture that was sitting in there. The actual diaphragm was separated, I wonder if maybe that's how the gas was getting past...it's like the diaphragm was 2 pieces...

I've bought that style in the states & did fine, BUT this one doesn't specify psi output. That style pump in the US comes in 2 pressures. Low is 2 to 4psi, and the higher is like 4 to 7psi. You MUST use a low pressure model, or add a pressure regulator.

I have had luck twice in the past very by carefully drilling out those screws. Go slow and do not let it heat up, at some point, you will see the other end move and then they will screw on out the other side.

Good advice, I just got them out. Now to find those little brass screws...